Harden OData error handling

[wilsonfreitas]

Closes #33

Summary

• Convert malformed OData service-root JSON, metadata XML/structure problems, and query JSON failures into contextual ODataErrors.
• Validate required OData service/query fields before downstream code indexes into responses.
• Add sync and async regression coverage for malformed OData payloads and missing fields.

Verification

• uv run pytest tests/test_odata.py tests/test_async.py -m "not integration" (38 passed)
• uv run pytest -m "not integration" (130 passed, 11 deselected)
• uv run ruff check bcb/ tests/
• uv run ruff format --check bcb/ tests/
• uv run mypy bcb/
• uv run --group docs sphinx-build -b html docs docs/_build/html